In general, a particle beam therapy system is provided with a beam generation apparatus that generates a charged particle beam; an accelerator that is connected with the beam generation apparatus and accelerates a generated charged particle beam; a beam transport system that transports a charged particle beam that is accelerated by the accelerator so as to gain predetermined energy and then emitted; and a particle beam irradiation system, disposed at the downstream side of the beam transport system, for irradiating an irradiation subject with a charged particle beam. Particle beam irradiation systems are roughly divided into a broad irradiation method in which a charged particle beam is enlarged in a scattering manner by a scatterer, and the shape of the enlarged charged particle beam is made to coincide with the shape of an irradiation subject in order to form an irradiation field; and a scanning irradiation method (the spot-scanning method, the raster-scanning method, and the like) in which an irradiation field is formed by performing scanning with a thin, pencil-like beam in such a way that the scanning area coincides with the shape of an irradiation subject.
In the broad irradiation method, an irradiation field that coincides with the shape of a diseased site is formed by use of a collimator or a bolus. The broad irradiation method is a most universally utilized and superior irradiation method where an irradiation field that coincides with the shape of a diseased site is formed so as to prevent unnecessary irradiation onto a normal tissue. However, it is required to create a bolus for each patient or to change the shape of a collimator in accordance with a diseased site.
In contrast, the scanning irradiation method is a high-flexibility irradiation method where, for example, neither collimator nor bolus is required. However, because these components for preventing irradiation onto not a diseased site but a normal tissue are not utilized, there is required a beam emission accuracy that is higher than the beam emission accuracy of the broad irradiation method.
With regard to a particle beam therapy system, there have been implemented various inventions that raise the accuracies of an irradiation position and an irradiation dose. Patent Document 1 discloses an invention, stated below, that has an objective of providing a particle beam therapy system capable of accurately irradiating a diseased site. In the invention disclosed in Patent Document 1, there are stored, in a memory device, the amount of charged particle beams scanned by a scanning apparatus and the position of a charged particle beam detected by a beam position detector while the charged particle beam is emitted; then, by utilizing the stored scanning amount and the beam position, the scanning amount of the beam scanning apparatus is set by a control apparatus, in accordance with the beam position based on information about a treatment plan. The relationship, between the scanning amount and the beam position, that is obtained by actually performing irradiation is stored in the memory device; therefore, accurate irradiation of a diseased site can be expected.
Patent Document 2 discloses an invention, stated below, that has an objective of providing a particle beam therapy system capable of ensuring high safety and emitting a charged particle beam in a highly accurate manner. In the invention disclosed in Patent Document 2, a charged particle beam emitted from a charged particle beam generation apparatus is supplied to a scanning electromagnet that performs irradiation on an irradiation plane perpendicular to the traveling direction of the beam; then, the amount of charged particle beams emitted from the charged particle beam generation apparatus is controlled based on the position and the dose, on the irradiation plane, of the charged particle beam that passes through the scanning electromagnet. Specifically, the supply of a charged particle beam to the region, among a plurality of regions formed by dividing the irradiation plane, where a target dose has been achieved is stopped, and charged particle beams are supplied to the other regions where the target dose has not been achieved. As described above, the irradiation dose in each of the regions is compared with the target dose, and the emission amount of a charged particle beam is on/off-controlled (supplied/not stopped), so that high safety is expected.
Patent Document 3 discloses an invention, stated below, with regard to a problem that hysteresis characteristics existing in the relationship between the current and the magnetic field of a scanning electromagnet deteriorates the accuracy of a beam irradiation position. The invention disclosed in Patent Document 3 has a first calculation means that calculates, without taking the effect of the hysteresis into account, the current value of a scanning electromagnet in accordance with the beam irradiation position based on an irradiation plan; and a second calculation means that performs, taking the effect of the hysteresis into account, a correction calculation of the current value of the scanning electromagnet calculated by the first calculation means. An irradiation control apparatus controls the current of the scanning electromagnet, based on the result of the calculation by the second calculation means. As described above, a correction calculation is performed by the second calculation means so as to eliminate the effect of the hysteresis, i.e., the second calculation means has a mathematical model where the hysteresis characteristics are represented, so that the improvement of the accuracy of a beam irradiation position is expected through the calculation.